1. Field of the Invention
The present invention relates to a "tie-up" decision device for an automatic transmission which decides whether or not "tie-up" has occurred. The "tie-up" is defined here that a state where engagement of an engagement side clutch proceeds too fast relatively to the release of a release side clutch, in case of executing a clutch-to-clutch gearshift in which the engagement of the engagement side clutch and the release of the release side clutch proceed concurrently. Also, the present invention relates to a gearshift control apparatus employing the tie-up decision device.
2. Description of the Prior Art
In case of executing any specified gearshift of an automatic transmission, two frictional engagement devices (clutches in a broad sense, each including a brake) must be often engaged and released at the same time, respectively (as a so-called "clutch-to-clutch gearshift"). On this occasion, unless the proceedings of the engagement and release of the respective frictional engagement devices are appropriately synchronized, the torque of the output shaft of the automatic transmission will lower, or the r.p.m. of an engine will blow up.
In such a gearshift control, therefore, it has heretofore been common practice to dispose a one-way clutch which effects a function being substantially equivalent to the function of either of the frictional engagement devices, with the intention of avoiding the drawback as stated above.
However, the method in which the engagement devices are synchronized using the one-way clutch in this manner incurs the problem that the cost of the automatic transmission rises in correspondence with the addition of the one-way clutch. Moreover, the weight of the automatic transmission increases, and the accommodation space thereof enlarges.
In view of such problems, in recent years, attempts to execute the "clutch-to-clutch gearshift" directly without employing the one-way clutch have been actively made again with a background being enhancements in the techniques of various sensors and enhancements in the electronic control techniques of hydraulic control systems.
By way of example, the official gazette of Japanese Patent Application Laid-open No. 37128/1990 discloses a method for coping with the present situation. Herein, the engine r.p.m. during a gearshift operation is monitored. When the engine r.p.m. has blown up (that is, when the engine speed has risen), it is recognized that the frictional engagement devices have fallen into an "underlap state" (in which the proceeding of the engagement lags relatively to that of the release). In contrast, when the engine r.p.m. has lowered, it is recognized that the frictional engagement devices have fallen into an "overlap state" (in which the proceeding of the engagement is too fast relatively to that of the release, and which shall hereinbelow be called the "tie-up" or "tie-up state" as stated at the opening of this specification).
Besides, as a method for deciding the tie-up state, the official gazette of Japanese Patent Application Laid-open No. 341527/1994 disclosesone wherein a time period taken since the beginning of a torque phase till that of an inertia phase is detected so as to decide tie-up when the time period is long.
With such a deciding method, really the blowing-up of the engine r.p.m. can be detected comparatively easily. Regarding the decision of the tie-up, however, only the occurrence of a very long (or intense) tie-up can be decided, that is, the precision of the decision is very inferior. Further, it is naturally impossible to accurately grasp the degree of the tie-up state. In performing a feedback control or a learning control, therefore, an appropriate control is very difficult. Especially in case of utilizing learning corrections in order to optimize the timings of a clutch pressure decrease/increase, the learning gain of the learning control in the direction of clearing the tie-up state cannot be optimized. Therefore, it occurs such a problem as a delay in the clearing of the tie-up state at the initial stage of learning, or as a hunting (the recurrence of slip states and tie-up states) at the later stage of the learning.
Moreover, when the tie-up state has occurred, the torque of the output shaft of the automatic transmission lowers suddenly to give rise to an intense gearshift shock. In some cases, very great load torques act on the individual members of the automatic transmission, and this forms a factor for degrading the durability of the automatic transmission.